1. Field of the Invention
The present invention relates to a device interposed between a telephone outlet and a telephone set, the device including a protected ringer whereby ambient sound cannot be transmitted back to the telephone outlet when the telephone set is on-hook.
2. Description of the Prior Art
In the present world of communications vast amounts of information and data is transmitted from one location to another via telephones. Concurrently with the increased use of telephones for information transmission has come the realization that many of the conversations or other sounds occurring in an area in which a standard telephone is located but not in use may be transmitted via the telephone ringer to the telephone company lines and thereafter detected by unauthorized persons having access to the lines. In particular, the small speaker or the electromechanical bell in the telephone used to produce the normal ring sound signals has the capability of being also used as a microphone even with the telephone handset on-hook, the speaker can transmit audible sounds (conversations or otherwise) emanating near the telephone set back out to the telephone lines with the possibility that unauthorized persons can determine the nature of the audible sounds.
The following references relate to systems for limiting access to or otherwise monitoring telephone sets: U.S. Pat. No. 4,393,268 to Miyoshi discloses a telephone security system wherein a unit is interposed between a telephone outlet and a telephone set to answer a call and respond only to the presence of a subsequently dialed-in security code; U.S. Pat. No. 4,392,023 to Sears discloses an off-hook telephone sensing device coupled to the telephone set for detecting the on-hook or off-hook condition of the telephone set the detected condition being utilized to multiplex between voice and data transmission; U.S. Pat. No. 4,396,805 to Wagner discloses a ring detector for a subscriber line interface circuit that includes a two-four wire hybrid circuit having an off-hook detector and a ring generator for producing an AC ring voltage of a given frequency, a circuit breaker being utilized by the telephone company to shut the ring signal off to avoid burn up of equipment in the telephone; U.S. Pat. No. 4,379,953 to Huff discloses a telephone set having a means for depressing a handset switch to prevent the telephone from becoming inoperative if left off the hook for a period of time; U.S. Pat. No. 3,474,196 to Bottos discloses a hand telephone having a switch which, when depressed, inactivates the transmitter portion of the handset; U.S. Pat. No. 4,006,316 to Bolgiano discloses a system for screening out undesired signals from desired signals; U.S. Pat. No. 4,143,250 to Simokat discloses a system for initiating a disconnect by remote signalling from the central office for fault isolation and testing to determine where the fault in the telephone system exists, the telephone company having the burden of initially isolating whether the subscribers equipment is at fault or whether the telephone equipment is at fault; U.S. Pat. No. 4,320,260 to Lechner discloses a circuit arrangement for determining the connection condition of a plurality of subscriber lines in a telecommunication network; U.S. Pat. No. 4,110,569 to Schindler et al discloses a system for determining the on/off-hook status of a telephone set; U.S. Pat. No. 4,152,549 to Ceruti et al discloses an apparatus for suppressing spurious ringing currents which may occur in the subscribers telephone set; and U.S. Pat. No. 4,021,617 to Jones et al discloses a telephone ringer isolator for isolating ringers and extending ringing range on a multiparty telephone line or for range extension only on single party lines.
Although the aforementioned references provide various systems relating to subscribers telephone sets, none of the systems described are directed to the problem of preventing ambient sounds which originate near the telephone set from being transmitted out over the telephone line when the telephone set is on-hook and detected by those not being authorized to have access to such sounds (in this sense, ambient sounds are defined to include, for example conversations and equipment generated noise). Therefore what is desired is to provide a relatively simple and cost effective system which allows a subscribers telephone set to be secured in a manner such that ambient sounds cannot be transmitted out through the telephone lines when the telephone set is in the on-hook status and to limit the transmission between the telephone company lines and the telephone set to audio signals when the telephone set is in the off-hook status.